A low molecular weight sulfated polysaccharide (L-A) was prepared by mild acid hydrolysis of crude fucoidan (F-A) from Laminaria japonica. In comparison with F-A, L-A had a lower proportion of fucose residues, but a similar proportion of sulfate. The galactose content of both fractions was relatively high, especially for L-A (57%). The antioxidant properties of the two polysaccharides were studied using two low-density lipoprotein oxidation systems. L-A had a stronger effect against low-density lipoprotein oxidation in both systems. F-A inhibited the AAPH-induced low-density lipoprotein oxidation, but had little effect on the CuCC-induced system due to its large molecular mass. The active sulfated fraction L-A, containing alactose, mannose and fucose (about 9: 2: 2) is reported here for the first time.